YZ400F backfires, wont idle, dies

Just curious, but is there a hose running from the head under the carb to a pull knob type valve bolted to your frame near the air boot?

You mean the hot start? if so yes it is there, I also pulled it off and verified that I can't blow air throw it when it's closed and I also verified the integrity of the hose that goes from the hot start to the motor. All good.

So I couldn't stand it any longer, it's been raining here for two weeks solid and today we got our first glimpse of blue sky. I jumped on it, with the 50 pilot jet in the carb and no real idle I said screw it and loaded up the bike and some tools and tore off to a local woods area to see what this thing had.

When I first got the bike strarted which was second kick I had some light pops and backfires as I sped away from the truck. As I continued to ride the bike seemed to perform better and the backfiring started to go away.

I had a couple stalls and it took me FOREVER to get the thing started again.. I was scared I was going to end up straneded with a fouled plug but eventually I remember "HOT START" DUH!! Once I figured that out it still took a few kicks but I got it going at every stall.

By the time I headed back to the truck the bike IDLED almost perfectly and there was almost no popping except for rapid deceleration but it was almost "normal" for any 4 stroke. At this point I'm pretty happy but I want to make sure this behavior is consistant and if it just needed to be ridden and cleaned out then that would be great. I'm getting the new header pipe tomorrow which I absolutely will put on and likely go back down to the stock jetting. When I have the tank off I'll pull the plug and see how it looks (lean / rich) and go from there.

When I was out riding I didn't make ANY adjustments to the idle or air screw, the airscrew is at 2 turns out exactly and the idle is I would say maybe half way turned in.

Oh an BTW TPS sensor is disconnected.

Well, success. I am very happy to say that my bike runs at 98.9% right now. I got the new header pipe and WOW! HUUUUGE difference. Much snappier on the throttle response, I can pull a wheelie in any gear now and the bike just wants to goooooo! I still have the 50 pilot in it and I did another 1/4 turn out on the pilot jet while riding today and it virtualy eliminated the popping on deceleration. The bike runs soooo smooth now, idles (purrrrs like a ktitten) it will idle all day long if I let it. I was actually THRILLED to see a glowing red header like I've heard so much about tonight! So my frustrations early on are all now laid to rest and this bike just frig'n screams! I really couldnt be happier. Gonna just fine tune the carb and check my plug to make sure I'm not running too rich or lean and then leave it alone.

My TPS sensor is still disconnected, I'm tempted to start playing with that but I dont want ti fix it broke if you know what I mean. How important is it to have that sensor connected anyway?

Thanks for all the advice guys, glad I think I got this resolved.

Doing a plup chop will not give an accurate indication of the air/fuel mix. If the bike pulls strong right up to the limiter, with no flat spot or hesitation at or near to 'WOT' - leave it be.

There are methods to finding the 'ideal' main#, plug colour is not one of them.

Have a look at the jetting data base & see how it goes.


Hey I'm new and this is my first post but cut a long story short, I have this same problem on my 1998 yz400, struggles to start and when it does it won't idle, backfiring on deceleration and in neutral, fuel is leaking from somewhere on the carb so I'm thinking something to do with the float/valve, I've read loads now and understand a number of things could be the problem, my main question is has anybody actually found a solution to this problem ? Also is the air/fuel screw located at the bottom of the bowl ? Because I can only see an idle control screw, appreciate any help, thanks.

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